I. Field of the Invention
The present invention relates generally to hydraulic systems and, more particularly, to significantly reducing fluid borne noise commonly present in such systems with particular reference to reduction of noise in vehicular power steering systems.
II. Description of the Prior Art
Fluid borne noise is commonly present in hydraulic systems powered by pumping apparatus such as gear, vane or piston pumps. Typically, the noise results when pressure waves are generated as pump flow ripple encounters system flow impedances. Flow ripple can be generated by the pumping apparatus as a combination of any, or all, of the following three types of disturbances:
1. Kinematic flow ripple, which is a function of pump geometry. By way of example, the flow output of a piston pump is generally a summation of an odd number of pistons moving in a sinusoidal manner. This results in flow ripple whose fundamental frequency is equal to the product of the pump's rotational speed and the number of pistons.
2. Compression flow ripple, which is a result of compression, or decompression, of a trapped fluid volume. Compression flow ripple is commonly encountered in gear pumps as the pump's gears mesh. This is because the gears mesh with a contact ratio greater than one whereby fluid can be trapped between succeeding sets of teeth whenever they are concomitantly in contact.
3. Leakage flow ripple, which is a result of pressure differentials across varying leakage paths formed between moving and stationary pump components.
Fluid borne noise present in a hydraulic system causes mechanical apparatus such as hydraulic lines, control valves, hydraulic motors, and supporting structural members to vibrate. In many cases such vibration is coupled to the atmosphere and is the source of objectionable acoustic noise. It is desirable to attenuate such fluid borne noise near a hydraulic system's pumping apparatus thereby isolating the rest of the system and minimizing vibration and resulting acoustic noise.
Fluid borne noise reduction apparatus of the prior art usually comprises a flexible metal tube, called a tuning cable, placed inside a section of volumetrically compliant hose. Such prior art apparatus is described in U.S. Pat. No. 3,323,305 entitled ATTENUATION DEVICE and issued to G. T. Klees in June 1967. Although such tuning cable designs are based upon destructive interference principles, their performance has never been fully analyzed. For instance, the automotive industry has resorted to empirical methods and subjective evaluations in applying the technology to vehicular power steering systems wherein its success has been marginal at best. The following quotation from SAE Technical Paper No. 931295 entitled ANALYSIS OF TUNING CABLES FOR REDUCTION OF FLUIDBORNE NOISE IN AUTOMOTIVE POWER STEERING HYDRAULIC LINES by M. C. Hastings and C. C. Chen given in May 1993 (which paper comprises mathematical techniques for analysis of the prior art technology) illustrates this point:
"The distributed parameter mathematical model does accurately predict attenuation of the pressure wave in simple (straight line) systems. In more complex systems, however, overall attenuation is a function of configuration, including the number of discontinuities and support points, and varies as a function of frequency. In real systems with many curves, bends and other discontinuities, the complex fluid-structure interaction dominates and tuning cables may have little, if any, effect on noise reduction in the frequency range of interest."
As a matter of fact, this analysis misses the mark slightly because the supposed destructive interference usually does not occur. This is for three reasons. Firstly, the pumping apparatus acts in the manner of a flow source rather than a pressure source in issuing the offending noise signal. Thus, it presents a substantially infinite source impedance to the tuning cable. Secondly, because the tuning cable is spiral wound with a discontinuous wall, it has distributed leakage therethrough. Thirdly, because the expandable nature of the volumetrically compliant hose, effective bulk modulus of fluid flowing therewithin is as much as 30 times smaller than the fluid itself. Thus, sound velocity and therefore wavelength are up to about 5.5 times less than within the tuning cable. In effect, prior art noise reduction apparatus dominantly comprises a distributed coupling of pressure to the volumetrically compliant hose which substantially acts like a continuous array of capacitors each coupled to ground.